Directional casing and liner drilling with mud motor

ABSTRACT

A directional casing drilling system includes a casing string, a mud motor operatively coupled to the casing string, a rotary steerable system operatively coupled to the mud motor, and a drill bit operatively coupled to the rotary steerable system.

BACKGROUND OF INVENTION

Wells are generally drilled into the ground to recover natural depositsof hydrocarbons and other desirable materials trapped in geologicalformations in the Earth's crust. A well is typically drilled using adrill bit attached to the lower end of a “drill string.” The drillstring is a long string of sections of drill pipe that are connectedtogether end-to-end. Drilling fluid, called “mud,” is typically pumpeddown through the drill string to the drill bit. The drilling fluidlubricates and cools the drill bit, and it carries drill cuttings backto the surface in the annulus between the drill string and the boreholewall.

In conventional drilling, a well is drilled to a selected depth, andthen the wellbore is typically lined with a larger-diameter pipe,usually called “casing.” Casing typically consists of casing sectionsconnected end-to-end, similar to the way drill pipe is connected. Toaccomplish this, the drill string and the drill bit are removed from theborehole in a process called “tripping.” Once the drill string and bitare removed, the casing is lowered into the well and cemented in place.The casing protects the well from collapse and isolates the subterraneanformations from each other.

Conventional drilling typically includes a series of drilling, tripping,casing and cementing, and then drilling again to deepen the borehole.This process is very time consuming and costly. Additionally, otherproblems are often encountered when tripping the drill string. Forexample, the drill string may get stuck in the borehole while it isbeing removed. These problems require additional time and expense tocorrect.

It is noted that a “liner” is a casing string that does not extend tothe top of a well. A liner is typically used when a well is drilled,cased, and then drilled again to deepen the well. The part of the wellthat is drilled past the initial casing string is cased with “liner.” Inpractice, the only difference between casing and liner is that the linerhas a smaller diameter, and it is suspended from the bottom of thecasing string above it. The difference between liner and casing is notgermane to the invention; thus, no distinction is made between casingand liner.

FIG. 1A shows a prior art drilling operation. A drilling rig 101 androtary table 104 at the surface are used to rotate a drill string 103with a drill bit 105 disposed at the lower end of the drill string 103.The drill bit 105 drills a borehole 107 through subterranean formationsthat may contain oil and gas deposits. Typically, an MWD (measurementwhile drilling) or LWD (logging while drilling) collar 109 is positionedjust above the drill bit 105 to take measurements relating to theproperties of the formation as the borehole 107 is being drilled.

To a person having skill in the art, MWD and LWD refer to differentthings. Typically, MWD relates to measurements about the orientation ofthe drill bit and the conditions in the borehole, where LWD refers tomeasurements of the formation and its contents. The distinction betweenMWD and LWD is not germane to the invention. In this description, MWD isused to refer either an MWD system or an LWD system.

The term “casing drilling” refers to using a casing string as a drillstring when drilling. Essentially, the larger diameter casing is used asa drill string to rotate the drill bit. A bottom hole assembly (“BHA”),including a drill bit, is connected to the lower end of a casing string,and the well is drilled using the casing string to transmit drillingfluid, as well as axial and rotational forces, to the drill bit. Casingdrilling enables the well to be simultaneously drilled and cased. Oncedrilling is completed, the casing is already in place and may then becemented.

FIG. 1B shows a prior art casing drilling operation. A rotary table 129at the surface is used to rotate a casing string 123 that is being usedas a drill string.

The casing string 123 extends downwardly into borehole 127. A drill bit125 is connected to the lower end of the casing string 123. Whendrilling with casing, the drill bit 125 must be able to pass though thecasing string 123 so that the drill bit 125 may be retrieved oncedrilling has been completed or when replacement or maintenance of thedrill bit 125 is required. Thus, the drill bit 125 is sized smaller thanthe inner diameter of the casing string 123.

The drill bit 125 drills a pilot hole 128 that must be enlarged so thatthe casing string 123 will be able to pass through the borehole 127. Anunderreamer 124 is positioned below the casing string 123 and above thedrill bit 125 so as to enlarge the pilot hole 128. A typical underreamer124 can be positioned in an extended and a retracted position. In theextended position, the underreamer 124 enlarges the pilot hole 128 tothe underreamed borehole 127, and in the retracted position (not shown),the underreamer 124 collapses so that it is able to pass through theinside of the casing string 123. FIG. 1B also shows an MWD collar 135positioned above the drill bit 125 and the underreamer 124, but belowthe casing string 123.

Casing drilling eliminates the need to trip the drill string and thencase the well. Instead, The drill bit may simply be retrieved by pullingit up through the casing string. The casing may then be cemented inplace, and then drilling may continue. This reduces the time required toretrieve the BHA and eliminates the need to subsequently run casing intothe well.

Another aspect of drilling is called “directional drilling.” Directionaldrilling is the intentional deviation of the wellbore from the path itwould naturally take. In other words, directional drilling is thesteering of the drill string so that it travels in a desired direction.Directional drilling is especially advantageous in offshore drillingbecause it enables many wells to be drilled from a single platform.Directional drilling also enables horizontal drilling through areservoir, which will increase the surface area of the well thattraverses the reservoir.

A directional drilling system may also be used in vertical drillingoperation as well. Often the drill bit will veer off of a planneddrilling trajectory because of the unpredictable nature of theformations being penetrated or the varying forces that the drill bitexperiences. When such a deviation occurs, a directional drilling systemmay be used to put the drill bit back on course.

One method of directional drilling uses a BHA that includes a benthousing (n and a mud motor. A bent housing 200 is shown in FIG. 2A. Thebent housing 200 includes an upper section 203 and a lower section 204that are formed on the same drill collar assembly, but are separated bya bend 201. The bend 201 is a surface adjustable mechanical joint in thedrill collar assembly.

With a bent housing 200, the drill string is not rotated from thesurface.

Instead, the drill bit 205 is pointed in the desired drilling direction,and the drill bit 205 is rotated by a-mud motor (not shown) located inthe BHA. A mud motor converts some of the energy of the mud flowing downthrough the drill pipe into a rotational motion that drives the drillbit 205. Thus, by maintaining the bent housing 200 at the same azimuthalposition with respect to the borehole, the drill bit 205 will drill inthe desired direction.

When straight drilling is desired, the entire drill string, includingthe bent housing 200, is rotated from the surface. The drill bit 205angulates with the bent housing 200 and drills a slightly overbore, butstraight, borehole (not shown).

Another method of directional drilling includes the use of a rotarysteerable system (“RSS”). In an RSS, the drill string is rotated fromthe surface, and downhole devices cause the drill bit to drill in thedesired direction. Rotating the drill string greatly reduces theoccurrences of the drill string getting hung up or stuck duringdrilling.

Generally, there are two types of RSS's—“point-the-bit” systems and“push-the-bit” systems. In a point-the-bit system, the drill bit ispointed in the desired direction of the borehole deviation, similar to abent housing. In a push-the-bit system, devices on the BHA push thedrill bit laterally in the direction of the desired borehole deviationby pressing on the borehole wall.

A point-the-bit system works in a similar manner to a bent housingbecause a point-the-bit system typically includes a mechanism forproviding a drill bit alignment that is different from the drill stringaxis. The primary differences are that a bent housing has a bend at afixed angle set on the surface and firmly coupled and aligned with thedrill string rotation, and a point-the-bit RSS has a bend angle that iscontrolled independently of the drill string rotation.

FIG. 2B shows a point-the-bit RSS 210. A point-the-bit RSS 210 typicallyhas an drill collar 213 and a drill bit shaft 214. The drill collar 213includes an internal orientating and control mechanism (not shown) thatcounter-rotates relative to the drill string. This internal mechanismcontrols the angular orientation of the drill bit shaft 214 relative tothe borehole (not shown).

The angle θ between the drill bit shaft 214 and the drill collar 213 maybe selectively controlled. The angle θ shown in FIG. 2B is exaggeratedfor purposes of illustration. A typical angle is less than 2 degrees.

The “counter rotating” mechanism rotates in the opposite direction ofthe drill string rotation. Typically, the counter rotation occurs at thesame speed as the drill string rotation so that the counter rotatingsection maintains the same angular position relative to the inside ofthe borehole. Because the counter rotating section does not rotate withrespect to the borehole, it is often called “geo-stationary” by thoseskilled in the art. In this disclosure, no distinction is made betweenthe terms “counter rotating” and “geo-stationary.”

A push-the-bit system typically uses either rotating or non-rotatingstabilizer and/or pad arrangement. The non-rotating stabilizer and/oractuated pad remains at a fixed angle (or geo-stationary) with respectto the borehole wall. When the borehole is to be deviated, an actuatorpresses a pad against the borehole wall in the opposite direction fromthe desired deviation. The result is that the drill bit is pushed oractuated in the desired direction.

FIG. 2C shows a typical push-the-bit system 220. The drill string 223includes rotating collar 221 that includes one or more extendable andretractable pads 226. When a pad 226 is extended into contact with theborehole (not shown) during drilling, the drill bit 225 is pushed in theopposite direction, enabling the drilling of a deviated borehole.

FIG. 3 shows a prior art drilling system that includes both casingdrilling and directional drilling. A rotary table 304 is used to rotatea casing string 311 that is being used as a drill string. A drill bit305 and an underreamer 313 are positioned at the lower end of the casingstring 311. The drill bit 305 drills a pilot hole 308 that is enlargedto an underreamed borehole 307 by the underreamer 313.

The casing drilling system also includes an RSS 315 that is positionedblow the casing string 311 and between the drill bit 305 and theunderreamer 313. The RSS 315 is used to change the direction of thedrill bit 305.

Nonetheless, a need still exists for an improved drilling system.

SUMMARY OF INVENTION

In one or more embodiment, the invention relates to a directionaldrilling system that includes a casing string, a mud motor operativelycoupled to the casing string, a rotary steerable system operativelycoupled to the mud motor, and a drill bit operatively coupled to therotary steerable system. In some embodiments, the casing drilling systemalso includes an underreamer positioned between the drill bit and thecasing string and operatively coupled to the casing string.

In other embodiments, the invention relates to a directional casingdrilling system that includes a casing string having an integral bendproximate a lower end of the casing string, a mud motor operativelycoupled to the casing string, and a drill bit operatively coupled to themud motor. In some embodiments, the directional casing drilling systemalso includes an underreamer positioned between the drill bit and thecasing string and operatively coupled to the casing string.

In one or more embodiments, the invention related to a method ofdirectional drilling that includes rotating a casing string at a firstspeed that is slower than an optimum drilling speed and operating a mudmotor to rotate a drill bit at a second speed that when summed with thecasing string at the first speed result in an optimum drilling speed.The method may then include changing the direction of the drill bit byoperating a rotary steerable system.

In other embodiments, the invention relates to a method of directionalcasing drilling that includes positioning a casing string so that a bendin a lower section of the casing string points in a desired azimuthaldirection and engaging a mud motor to rotate a drill bit.

Other aspects and advantages of the invention will be apparent from thefollowing description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1B shows a prior art drilling operation.

FIG. 1B shows a prior art casing drilling operation.

FIG. 2A shows a prior art bent housing.

FIG. 2B shows a prior art “point-the-bit” system.

FIG. 2C shows a prior art “push-the-bit” system.

FIG. 3 shows a prior art directional casing drilling operation.

FIG. 4 shows a directional casing drilling system in accordance with oneembodiment of the invention.

FIG. 5 shows a directional casing drilling system in accordance withanother embodiment of the invention.

FIG. 6 shows a directional casing drilling system with a casing shoecutter, in accordance with one embodiment of the invention.

DETAILED DESCRIPTION

In some embodiments, the invention relates to a directional casingdrilling system with a mud motor located near a rotary steerable systemdisposed inside the casing string. In other embodiments, the inventionrelates to a method of directional drilling with casing.

FIG. 4 shows a directional casing drilling system in accordance with oneembodiment of the invention. A bottom hole assembly 430 (“BHA”) ispositioned below a casing string 411 that is used as a drill string. Itis noted that in some embodiments, some of the components of the BHA 430may be positioned in a different order than is shown in FIG. 4 (asdescribed below), and some components may be disposed inside the casingstring 411. The exact position of each component of the BHA 430 is notintended to limit the invention.

The casing string 411 transmits rotary motion and downward force (calledweight-on-bit or “WOB”) to a drill bit 405 positioned below the lowerend of the casing string 411. The drill bit 405 drills a pilot hole 408that is enlarged by an underreamer 413 to a size that will enable thecasing string 411 to pass through the borehole 407. The drill bit 405and the underreamer 413 are part of a BHA 430 that is rotationally andaxially coupled to the casing string 411. In one embodiment, the BHA 430is attached to the casing string 411 via an articulating casing latch412. The articulating casing latch 412 may be disengaged so that the BHA430 can be retrieved through the casing string 411 while leaving thecasing string 411 in place.

In this embodiment, the BHA 430 includes the underreamer 413, an MWDdevice 415, a mud motor 417, a rotary steerable system (“RSS”) 419, andthe drill bit 405. The RSS 419 may be any type of RSS, such as apoint-the-bit system or a push-the-bit system. There are many types ofRSS devices that are known in the art. The type of RSS device is notintended to limit the invention.

Each of the sections in the BHA 430 are “operatively coupled” to thecasing string 411. In this disclosure, operatively coupled meansconnected in such a way that the items may operate together. Forexample, in FIG. 4, the drill bit 405 is operatively coupled to thecasing string 411 by way of the RSS 419, the mud motor 417, the MWDdevice 415, the underreamer 413, and the casing latch 412. Thus,operatively coupled does not necessarily mean directly connected, itsimply means connected in such a way that the devices may operatetogether.

The rotary table 404 at the surface rotates the casing string 411 at afirst speed that is sufficient to reduce the forces of sliding frictionbetween the borehole wall 407 and the casing string 411. In someembodiments, the rotational speed of the casing string 411 is notsufficient to effectively carry out drilling operations. The mud motor417 is operatively coupled between the casing string 411 and the drillbit 405, and it is used to augment the rotational speed of the drill bit405 and the RSS 419. A mud motor, such as the one shown at 417, is adevice that is well known in the art. A mud motor converts some of thefluid energy in the mud flow into rotary motion. Thus, the mud motor 419causes everything below it (e.g., the RSS 419 and the drill bit 405 inFIG. 4) to rotate with respect to the rest of the drilling system.

There are several significant differences in using casing as a drillstring as opposed to using normal drill pipe; these differences arerelated to the larger diameter of the casing. Conventional drilling usesa drill string (e.g., drill string 103 in FIG. 1) that has a diametermuch smaller than the diameter of the borehole (e.g., 107 in FIG. 1).Casing drilling, on the other hand, uses a casing string (e.g., casingstring 411 in FIG. 4) as a drill string. The casing string has adiameter that is very close to the diameter of the borehole. Thus, thecasing string 411 experiences more frictional contact with the boreholewall.

Another factor in drilling with casing is that the larger diametercasing will have much more bending stress and lower fatigue limits whenrotated at the same speed as normal drill pipe for a given boreholeradius of curvature otherwise known as “dog-leg” severity or DLS. Evenwhere the casing has the same wall thickness as a normal sized drillstring, the casing will be subject to higher bending stress because thecasing wall rotates farther away from the axis of rotation. Thus,bending stress, fatigue, and wear concerns are greater for a casingdrilling system.

Any rotation of the casing string 411 will significantly reduce thefrictional forces that will be experienced between the casing string 411and the borehole wall 407. The reduced friction enables the casingstring 411 to be more easily moved through the borehole 407. In order toreduce the bending stress, fatigue, and wear concerns required tooperate the system, in some embodiments of the invention, the casingstring 411 is rotated at a speed that is slower than is optimal fordrilling. To compensate for the slower speed of rotation, as will bedescribed below, the mud motor 417 is used to augment the rotationalspeed of the drill bit 405.

A mud motor 417 converts fluid energy in the mud flow to rotationalmotion. The energy that is saved by not rotating the casing string 411at full speed will offset to some degree the energy that is consumed inthe mud motor 417. The mud motor 417 will increase the rotational speedof the RSS 419 and the drill bit 405 to a speed that is sufficient oroptimum for drilling.

In some embodiments, an MWD device 415 is included in the BHA 430 tomeasure the inclination and azimuthal angle of the BHA 430. The MWDdevice 415 enables a driller at the surface to monitor the position anddirection of the drill bit 405 so that the driller can make changes whendesired. As shown in FIG. 4, the MWD device 415 may be located above themud motor 417. This enables communications with the MWD device 415 viamud pulse telemetry.

Additionally, the MWD device 415 may be located below the underreamer413 so that the MWD device 415 is still positioned as close as possibleto the drill bit 405. The exact position and order of equipment in theBHA is not intended to limit the invention. Those having ordinary skillin the art will be able to devise other configurations of a BHA that donot depart from the scope of the invention.

FIG. 5 shows a bent housing casing drilling system. The bent housing isintegral with the casing string 511. The lowermost section of the casingstring 511 includes a permanent bend 525 near the bottom of the casingstring 511. The drilling system also includes a BHA 530 that is that isoperatively coupled to the casing string 511 by a casing latch 512. Inthe embodiment shown in FIG. 5, the MWD device 515 and the mud motor 517are disposed inside the lower section of the casing string 511. Thelower section of the casing string 511 protects the MWD device 515 andthe mud motor 517 in the event of hole collapse or other problem in thehole and could help facilitate their removal. The bottom of the casingstring 511 also includes an offset centralizer 519 that is used toradially position the BHA with respect to the casing string insidediameter.

As with other casing drilling systems, the drill bit 505 drills a pilothole 508 that is enlarged by an underreamer 513 so that the casingstring 511 will be able to move downward through the borehole 507. Inthe configuration shown in FIG. 5, the underreamer 513 and the drill bit505 are disposed below the casing string 511, and the mud motor 517 andthe MWD device 515 are disposed inside the lower section of the casingstring 511. This arrangement is not intended to limit the invention.

The bend 525 in the casing string 511 enables a driller to control thedirection of drilling. The casing string 511 is positioned so that thebend 525 causes the drill bit 505 to point in the desired direction. Thecasing string 511 is not rotated with respect to the borehole, so thatthe drill bit 505 will continuously point in the desired direction. Themud motor 517, which converts fluid energy to rotational motion, drivesthe rotation of the drill bit 505. Thus, the drill bit 505 is rotatedeven when the casing string 511 is not rotated. As drilling progresses,the drill bit 505 will turn in the desired direction. It is noted thatthe angle shown in FIG. 5 is much larger than a typical bent housing.This is done for illustration purposes. A typical bend is around onedegree, although larger or smaller angles may be used without departingfrom the scope of the invention.

In order to drill along a straight path, the casing string 511 isrotated from the surface. The rotation will cause the bend 525 in thecasing string 511 to rotate through 360°, thereby eliminating thedirectional bias caused by the bend. Because of the rotation, the drillbit 505 will drill a straight, but slightly overbore, pilot hole 508.The underreamer 513 will also drill a slightly overbore hole 507.

In some embodiments, the casing string 511 is rotated at a speed that isslower than the optimal drilling speed. The mud motor 517 may be used toincrease the rotational speed of the drill bit 505 and underreamer 513.By doing so, the casing string 511 will experience less friction withthe borehole wall, while still limiting the amount of rotation in thecasing string 511.

In some embodiments, the lower section of the casing string 511 isconstructed of non-magnetic and/or composite non-metallic material. Forthose embodiments that include an MWD device 515 or an LWD device (notshown), in the BHA 530, the non-magnetic material will enable the deviceto magnetically survey and evaluate the formations that are drilled.

FIG. 6 shows a casing string 611 that may be used with certainembodiments of the invention. The BHA 630 includes, in this embodiment,a mud motor 613, an RSS 619, and a drill bit 605. In some embodiments,the BHA 630 may also include an MWD device (not shown). The BHA 603 maybe coupled to the casing string 611 by an articulating casing latch 612.The BHA 630 does not include an underreamer. Instead, the bottom end ofthe casing string 611 comprises a casing shoe cutter 641.

The casing shoe cutter 641 is a cutting surface at the bottom of thecasing string 611 that enlarges the pilot hole 608 drilled by the drillbit 605 to a size that will enable the casing string 611 to pass throughthe borehole 607. The casing shoe cutter may be any type of cuttingsurface known in the art. For example, the casing shoe cutter 641 may becoated with a superhard material, such a polycrystalline diamond ortungsten carbide. Whether a casing drilling system uses an underreamer,a casing shoe cutter, or another device to enlarge a pilot hole is notintended to limit the invention.

The various embodiments of the invention may present one or more of thefollowing advantages. A casing drilling system enables casing to beinserted into a well during the drilling process. When drilling iscompleted or suspended, the casing is already in place. This saves thetime and expense of having to run casing into a well subsequent to thedrilling process, and the casing also prevents the borehole wall fromcollapsing into the borehole during the drilling operation.

Advantageously, a casing drilling system with a rotary steerable systemand a mud motor enables the casing string to experience reduced frictionwith the borehole wall while also reducing the amount of casing rotationrequired to operate the drilling system. By using the mud motor, thedrill bit may be rotated X at a faster speed than the casing string sothat drilling will be more efficient, along with a reduction in casingwear and fatigue problems.

Advantageously, a casing drilling system with a bent housing integralwith the lower section of the casing string will enable directionaldrilling using a bent housing. The casing will then pass through thedrilled borehole so that the borehole is protected against collapse.

Advantageously, a casing drilling system having an MWD device positionedabove a mud motor will enable communication with the MWD device throughmud pulse telemetry. It is noted that, in addition to mud telemetry, theMWD device may also communicate with computers at the surface viaelectrical or sonic means. Also, an casing string that is constructed ofa non-magnetic and/or non-metallic composite material will enable theuse of LWD devices that are positioned inside the lower end of thecasing string.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

1. A directional casing drilling system, comprising: a casing string; amud motor operatively coupled to the casing string; a rotary steerablesystem operatively coupled to the mud motor; and a drill bit operativelycoupled to the rotary steerable system.
 2. The directional casingdrilling system of claim 1, further comprising an underreamer disposedbelow the casing string and above the drill bit, and operatively coupledto the casing string.
 3. The directional casing drilling system of claim1, further comprising a casing shoe cutter disposed at a bottom end ofthe casing string.
 4. The directional casing drilling system of claim 1,wherein the rotary steerable system comprises a push-the-bit system. 5.The directional casing drilling system of claim 1, wherein the rotarysteerable system comprises a point-the-bit system.
 6. The directionalcasing drilling system of claim 1, further comprising a measurementwhile drilling collar disposed above the mud motor and operativelycoupled to the casing string.
 7. The directional casing drilling systemof claim 1, further comprising an articulating casing latch.
 8. Adirectional casing drilling system comprising: a casing string having anintegral bend proximate a lower end of the casing string; a mud motoroperatively coupled to the casing string; and a drill bit operativelycoupled to the mud motor.
 9. The directional casing drilling system ofclaim 8, further comprising an underreamer positioned between the drillbit and the casing string and operatively coupled to the casing string.10. The directional casing drilling system of claim 8, wherein the mudmotor is disposed inside the lower end of the casing string.
 11. Thedirectional casing drilling system of claim 8, further comprising ameasurement-while-drilling device.
 12. The directional drilling systemof claim 11, wherein the measurement-while-drilling device is positionedabove the mud motor and coupled to the casing string.
 13. Thedirectional drilling system of claim 8, further comprising an offsetcentralizer disposed inside the casing string at the lower end of thecasing string.
 14. A method of directional drilling, comprising:rotating a casing string at a first speed that is slower than an optimumdrilling speed; operating a mud motor to rotate a drill bit at a secondspeed; and changing the direction of the drill bit by operating a rotarysteerable system.
 15. The method of claim 14, further comprisingenlarging a pilot hole drilled by the drill bit using an underreamercoupled to the casing string.
 16. The method of claim 14, wherein abottom end of the casing string comprises a casing shoe cutter andfurther comprising enlarging a pilot hole drilled by the drill bit usingthe casing shoe cutter.
 17. The method of claim 14, further comprising:measuring a drill bit azimuth and a drill bit inclination; and adjustinga drilling direction based on at least one of the drill bit azimuth andthe drill bit inclination.
 18. A method of directional casing drilling,comprising: positioning a casing string so that a bend in a lowersection of the casing string points in a desired azimuthal direction;and engaging a mud motor to rotate a drill bit.
 19. The method of claim18, further comprising: measuring a drill bit azimuth and a drill bitinclination; and repositioning the casing string based on at least oneof the drill bit azimuth and the drill bit inclination.
 20. The methodof claim 18, further comprising: measuring a drill bit azimuth and adrill bit inclination; determining when the drill bit is pointed in adesired direction from at least one of the drill bit azimuth and thedrill bit inclination; and drilling a straight path by rotating thecasing string.